1. Field of the Invention
The present invention relates to a power converter preferably applied to a controller for controlling an electric motor which is used as a drive source of a rolling stock, for example, and more specifically, relates to a power converter comprising an inverter, an AC load receiving electric power from the inverter, and a battery connected in parallel with the inverter.
2. Description of the Related Art
Inverters widely used in the industrial world mainly adopt a system for converting DC electric power to AC electric power via variable voltage variable frequency (VVVF) control using a pulse width modulation (PWM) system.
For example, in the field of rolling stocks, a variable voltage variable frequency (hereinafter abbreviated as VVVF) inverter is used as a system for driving the AC motor of the rolling stock. The rolling stock is accelerated by driving the electric motor via the VVVF inverter, and a regeneration brake control is performed widely in which the electric motor is operated as a generator during braking operation so as to convert kinetic energy into electric energy and to return the same to an overhead trolley (hereinafter referred to as overhead cable).
Recently, an art of applying the battery to effectively utilize the regeneration electric power obtained via regeneration braking and to realize further energy saving has been studied and put to practical use.
For example, patent document 1 teaches an art of driving the rolling stock highly efficiently by connecting a battery in parallel with an input side of an inverter in a power converter, so as to store the regeneration energy and effectively utilize the same.
Further, patent document 2 discloses an art in which a battery is connected in series to a ground side of the input of the inverter so as to elevate the input voltage of the inverter, according to which the characteristic area of the high speed side of the electric motor is expanded and the performance of regeneration braking acquiring braking force by consuming the rotational energy of the electric motor or of power running for driving the rolling stock is enhanced.
The VVVF inverter is also used in inverter air-conditioners and the like in the field of home electrical appliances.
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2009-89503
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2008-278615
Non-Patent Document 1: Technical Data on Lithium-Ion Battery, Sanyo Electric Co., Ltd (2002) (URL: http://www.orient-technology.com/Sanyo/technot/lion_J.pdf)
Non-Patent Document 2: Power Electronics, Takao Hirasa, (p. 83-85), Kyoritsu Shuppan Co., Ltd (1992)
According to a controller of a rolling stock equipped with a battery, during reduction of regeneration load, the regeneration electric power can be absorbed and stored in the battery, so that meaningless consumption of kinetic energy of the rolling stock can be prevented, and simultaneously, the consumption of air brake and the like due to reduction of regeneration brake force can be cut down.
On the other hand, if a power storage battery is used as the battery, the power storage battery has a drawback in that generally, repeated discharge and charge thereof causes resolution of the electrolytic solution, which leads to characteristic degradation of the power storage such as the increase of internal resistance or the deterioration of battery capacity. Further, it is known that the speed of degradation is accelerated by over-charge and over-discharge of the battery or by the increase of temperature accompanying the same (refer to non-patent document 1).
The discharge and charge of power of the battery is controlled via a power converter such as a DC-DC converter. Such a power converter is generally composed of a power semiconductor device, wherein the power semiconductor device is subjected to switching control so as to control the electric power being discharged from or charged to the battery, so that harmonic distortion is contained therein, similar to the aforementioned VVVF inverter. The ripples contained in the discharge and charge currents or the repeated precipitous change of currents lead to accelerated degradation of the battery, so that suppression thereof is desired.
Known methods for suppressing the ripples superposed via the power converter generally relate to increasing the capacity of the reactor arranged within the power converter and increasing the switching frequency of the power semiconductor device (refer to non-patent document 2).
On the other hand, the power storage capacity of the battery disposed in the rolling stock is set so as to absorb all the regeneration electric power generated via regeneration braking, but since the mounting area under the floor of the vehicle is limited, it is preferable that a battery with a minimum battery capacity is mounted on the vehicle.
However, the reactor within the power converter has a high capacity and weight, and the increase in the size thereof is not desirable since the area under the floor of the vehicle is limited. It is also not desirable to increase the switching frequency of the power semiconductor device since the switching loss causes heat generation.